From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marc Zyngier <marc.zyngier@arm.com>
Subject: Re: [kvm-unit-tests PATCH] arm64: add micro test
Date: Thu, 3 May 2018 17:24:24 +0100
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Cc: pbonzini@redhat.com,
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To: "Kalra, Ashish" <Ashish.Kalra@cavium.com>,
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On 03/05/18 12:12, Kalra, Ashish wrote:
> 
>>>
>>> The reason for the problem you're seeing is probably that the
>>> constraints are not quite right. Here, x0 is not simply clobbered, but
>>> is actively written to in the middle of the sequence (it is at least an
>>> early clobber). It is also, I assume, a result from the hypercall, so it
>>> cannot simply be discarded.
>>>
>>> It would help to get a disassembly of the function, but I'd tend to
>>> rewrite the code as such:
>>>
>>> extern int bar(void);
>>>
>>> int foo(void)
>>> {
>>> 	register int w0 asm("w0");
>>> 	int a, b;
>>>
>>> 	a = bar();
>>> 	w0 = 0x4b000000;
>>> 	asm volatile("hvc #0" : "+r" (w0) :: );
>>> 	b = bar();
>>>
>>> 	return a - b;
>>> }
> 
> There still is the issue of x1-x3 registers being used for local 
> variables and clobbering of the same across the "hvc" call.
> 
> Comparatively, the fix below works more reliably :
> 
> asm volatile("hvc #0" : "+r" (w0) :: "x1","x2","x3");

Is that because this hypercall is explicitly clobbering these registers?
or because KVM is corrupting them? If the former, fine. If the latter,
that'd be a KVM bug.

So which one is it?

	M.
-- 
Jazz is not dead. It just smells funny...